Roller welding machines are appliances for welding metal sheets along edges or along junctures with a continuous process during which a roller that passes over the pieces to be welded is used as electrode. These machines have a pair of electrodes installed on both sides of the plates to be welded directed along the thickness of the plates, the opposed faces of the two plates being welded one to the other for constituting a welded portion. Each electrode is sized so as to convey a relatively strong welding current throughout the plates to be welded.
The contact resistance between the plates to be welded is generally much greater than the electric resistance of electrodes, than that of the plates themselves and than the contact resistances between electrodes and plates, thus the welding current will generate heat for Joule effect essentially in correspondence of the interface between the two plates that will melt and, when pressed one against the other by two rollers (anode and cathode) or by the roller and the plane electrode (anode and cathode), will weld one to the other.
Typically, roller welding machines comprise:                a welding head having an anode (or cathode) roll, typically made of copper, destined to be crossed by a relatively great current;        a cathode (or anode) electrode that may be circular or planar, typically made of copper, destined to be connected to the pole opposite to that of the anode roller. In case of a rotating electrode, it will be counter-rotating in respect to the anode roller so as to follow the forward motion of the two plates to be welded during the welding operation. In case of a planar electrode, its function is to act as a static bearing for ensuring the electric contact needed for current flowing;        elastic means for pulling the anode roller towards the cathode electrode so as to press the two plates to be welded placed between the rollers or between a roller and a planar electrode.        
Typically the welding head has a stator, electrically connected with a voltage generator, on which a cave shaft is pivotally mounted by means of ball bearings. The anode roller is fixed to the cave shaft and is cooled by contact with a ball bearing flange that has communicating grooves with the inner cavity of the shaft so as to define inner channels for circulating a coolant, so as to prevent the roller from overheating and melting the surface of the plate with which it comes into contact. Substantially, the cooled anode roller has thermal dissipation properties better than those of the material to be welded in order to dissipate heat on the surface of the seam joint. From a thermal point of view, the anode roller sinks heat from the joint so as to attain thermal equilibrium.
The shaft may eventually be connected through gears to a motor for making it rotate, so as to have, with appropriate appliances, the constant control of the peripheral speed of the roller in all phases of the welding path and the consequent regulation of the remaining welding parameters (pull force between the electrodes, welding periods, etc.). An eventual motorization of the anode roller allows to prevent micro-sliding and bonding of the same during the welding.
A typical constructive problem of the welding heads is the fact that the stator is connected to a current generator and it is necessary to provide electric connections at low resistance for transferring relatively high currents to the anode roller.
It is well known in the art to use sliding contacts, fixed to the stator, pressed against the cave shaft so as to minimize contact resistance. Typically, these sliding contacts are placed against the circumference of the shaft and are pulled against the shaft along a radial direction.
An drawback of these radial sliding contacts is the fact that they are subjected to wear and tear and it is necessary to disconnect the shaft from the stator in order to proceed to their substitution.
It is also known the use of a cave shaft of the type shown in FIG. 1, belonging to a welding head of roller welding machine made by Soudronic. It is shaped so as to have a pair of opposed sliding annuli 2 that surround the body 1 of the cave shaft, inside which the coolant flows. Respective rings 3 of the type shown in the frontal view of FIG. 2 and in the back view of FIG. 3, are pressed against both sides of the annuli 2. These rings are composed of:                a metal plait 6, typically made of copper, for providing current;        an external metal flange 4a and a corresponding external metal counter-flange 4b, with fixing holes 5 to the stator of the welding head, that together hold an external portion of the metal plait 6;        a ring shaped sliding element 7, typically made of silver, destined to be placed in contact with a respective annulus 2;        an inner metal flange 8, shown in FIG. 3, that cooperates with the ring shaped sliding element 7 for sandwich holding the metal plait 6.        
The metal plait 6 is thus a flexible mechanical constraint between the outer flanges 4a and 4b, fixed to the stator of the welding head, and the sliding ring shaped element 7, that follows eventual longitudinal motion of the cave shaft and is always in contact with the annuli 2 as far as the ring-shaped element wears.
As shown in FIG. 3, the metal plait 6, the inner metal flange 8 and the sliding ring shaped element 7 are fixed together by means of a zigzag welding 9, in order to prevent tangential forces during the sliding make the sliding ring shaped element 7 move in respect to the metal plait 6.
A drawback of such a system consists in that the shown shaft is fabricated by lathe-shaping a single block of copper so as to define the annuli 2 and the body 1. This operation make its manufacturing onerous because the annuli 2 must be relatively large so as to have a small resistance to current conduction, thus time and production costs of the piece shown in FIG. 1 are relatively high.
Moreover, realizing the sliding contact 3 is a relatively expensive operation because it is relatively difficult to realize the zigzag welding 9.
Finally, as for the welding heads with radial sliding contacts, it is required to dismount the shaft from the stator for unscrewing the rings 3, that are periodically replaced because the silver sliding element 7 wears during the use. Also the shaft, because of friction with the sliding element 7, is subjected to wear and this may lead to the replacement of the shaft, even if less frequently than the ring shaped sliding contact 3.
In order to obviate to these problems, there are welding heads without sliding contacts wherein there is a gap between the stator and the rotor that defines an enclosed space that is filled with a current conducting liquid, typically mercury. Even if the enclosed space is nominally waterproof sealed, there are always small leakages due to use, thus current conducting liquid shall be periodically added.
This solution allows to avoid the use of sliding contacts, though it has been forbidden by laws because mercury is toxic.